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+# Steam multiplayer — three-machine test procedure
+
+The full end-to-end: three machines, three Steam accounts, one lobby,
+one marshaled race over Steam Datagram Relay. Written 2026-07-12 for
+the first live test.
+
+## Per machine (all three)
+
+1. Steam client installed, logged in (different account per machine),
+   and RUNNING.
+2. Copy the `dist\` folder onto the machine.
+3. In the dist folder, create `steam_appid.txt` containing exactly:
+   `480`
+   (Spacewar, Valve's public dev AppID — replaced by the real AppID
+   once the store page exists.)
+4. Add to `environ.ini`:
+   `RP412STEAM=1`
+5. Start with `start-windowed.bat` as usual.
+
+Sanity check per machine: `rpl4.log` should show
+`SteamNetTransport: up as 169.254.x.y (fake ports N console, M game)`.
+If it says "staying on TCP", Steam isn't running / not logged in / the
+appid file is missing.
+
+## The test
+
+1. **Machine A (host):** pick a track, vehicle, color, pilot name on
+   the setup menu, then click **HOST STEAM RACE**. The lobby room
+   appears with A listed as `[HOST]`.
+2. **Machines B and C:** click **JOIN STEAM RACE** on the setup menu.
+   (It joins the first open RP412 lobby it finds — pick loadouts on the
+   menu BEFORE joining; the lobby publishes them.)
+3. When all three names show bright in A's room (bright = FakeIP
+   published), A clicks **L A U N C H  R A C E**.
+4. All three pods leave the room and boot the mission. Expect in logs:
+   - every pod: `SteamNetTransport: peer registered` x3
+   - members: `SteamNetTransport: listening on engine port 1501/1502`
+   - A: `LocalConsole: connecting to pod 169.254...` then
+     `egg sent`, `EGG ACK (mesh complete)` per member
+   - everyone: `All connections completed!` then the race
+5. The race runs for the length A picked; A's console stops everyone;
+   A gets the results screen with all three personas and scores.
+6. Everyone lands back in the lobby room. A can launch again —
+   different track, same lobby, no restarts.
+
+## Notes and knowns
+
+- All pods use engine ports 1501/1502 by convention under Steam; the
+  transport maps them to the Steam-assigned fake ports per peer.
+- FakeIPs are allocated per session — they are exchanged live through
+  lobby member data, never configured by hand.
+- Same-LAN machines may get a direct route from SDR; different
+  networks relay. Both are fine.
+- Two instances under ONE Steam account cannot mesh (one identity per
+  account) — hence three machines.
+- If a member joins after A already launched once, it will answer the
+  NEXT launch only (launch signals are nonced).
+
+## What to collect if something breaks
+
+`rpl4.log` from each machine (each dist folder), plus which step of
+the sequence above diverged.